As is known, a phase detector is a device that measures the phase difference between two periodic input signals of the same frequency. A bang-bang phase detector is a variant of the phase detector, where the output of the detector merely indicates which of the two input signals arrived earlier in time.
As is also known, a phase and frequency detector (PFD) is a device that measures the phase difference between two periodic input signals when the two input signal are not of the same frequency.
It is also known that phase detectors and PFDs are typically used in phase locked loops (PLL) or delay locked loops (DLL) to generate an error signal that is used by a filter inside a loop.